The invention relates to a system for activating a device in a well.
After a well has been drilled and casing has been cemented in the well, one or more portions of the casing adjacent pay zones are perforated to allow fluid from the surrounding formation to flow into the well for production to the surface. Perforating guns may be lowered on a tubing string into the well and the guns fired to create openings in the casing and to extend perforations into the surrounding formation.
Several firing techniques are available for firing perforating guns, including percussion-, pressure-, and electrically-actuated systems. Referring to FIGS. 5A-5C, in one type of system having a percussion-actuated firing head 401, a drop bar 400 (which is a cylindrical weight or sinker bar) is dropped down the tubing string from the surface or lowered downhole with a slick line. The drop bar 400 strikes a percussion-type detonator 402 in the gun firing head. As a safety feature, the drop bar contains a firing pin 404 that automatically retracts (FIG. 5C) at a set time after impact (FIG. 5B). As the firing pin 404 retracts, the drop bar 400 comes to rest on a guide stop 406 in the firing head 401 to prevent the drop bar 404 from contacting the percussion detonator 402. As a result, if the perforating guns do not fire for any reason after impact, the retracted firing pin on the drop bar 400 cannot impact the percussion detonator 402 to fire the guns when the gun string is being retrieved from the well.
Referring to FIG. 6, in a pressure-actuated firing system, differential pressure is used to fire the perforating guns. Pressure to actuate the firing head 501 is applied down the annulus between the tubing above the firing head and tubing packer (not shown) and the casing 501. The differential-pressure firing head 506 utilizes a flow tube 502 through the production tubing packer to transfer annulus pressure above the packer to an isolated release piston 503 in the firing head 506 located beneath the packer. The release piston 503 is held in place by shear pins 504. The annulus pressure above the packer is applied in region 511 against the top surface of the release piston 503. Fluid pressure from the rathole 513 (the region of the well beneath the packer) is transmitted through slots 505 into a chamber 512 under the piston 503.
When the annulus pressure exceeds the rathole pressure by a predetermined amount, the differential pressure causes the release piston 503 to break the shear pins 504 and to drive a firing pin 507 into a percussion cap 509, which then initiates a detonating cord 510 to fire the perforating gun. The safety features in such a pressure-actuated firing system include the shear pins that lock the release piston 503 until sufficient differential pressure is applied from the surface to break the shear pins and move the release piston.
In an electrically-actuated firing system, an actuating electric current is transmitted along an electrical conductor connected between the firing head and an electric power source at the surface. The electrical conductor can be run in a wireline or through a coiled tubing. FIG. 7 illustrates an example of an electrical line 34a run through a coiled tubing 44a. The coiled tubing 44a is connected to a coiled tubing logging head 46a, which in turn is connected to a deployment bar 50a. The firing head of the perforating system is located in the deployment bar 50a and includes an electrical detonator 102a. The electric line 34a runs from the surface through the coiled tubing logging head 46a and the deployment bar 50a to the detonator 102a. Electric current in the electric line 34a activates the electrical detonator 102a in the firing head to initiate a detonating cord 104a that extends to the perforating gun 52a. In the electrically-actuated system depicted in FIG. 7, electrical connection and ballistic connection (between the detonator 102a and the detonating cord 104a) have both been made before the perforating tool is lowered downhole. As long as safety procedures are strictly followed to ensure that the surface electric source is not activated while the gun string is being lowered, inadvertent firing of the gun may be avoided.